1. Field of the Invention
This invention relates to a wet friction material that constitutes a single friction plate or plural friction plates for use in a clutch for an automatic transmission, which may be hereafter referred to as “AT”, of a vehicle, an automatic transmission of automobiles or motorcycles, or the like.
2. Description of the Related Art
In recent years, high fuel efficiency in a vehicle and a low drag torque characteristic are required from the viewpoints of a rise in crude oil price and prevention of global warming.
Generally, oil grooves are formed on a contact surface side of segment pieces with a counter plate and lubricating oil or automatic transmission fluid, which may be hereafter referred to as “ATF”, is supplied thereto to lessen a drag torque. “ATF” is a registered trademark of Idemitsu Kosan Co., Ltd. However, the lubricating oil for automatic transmission or the automatic transmission fluid is generically abbreviated here as “ATF”, regardless of the registered trademark.
A conventional wet friction material has reduced the drag torque by controlling a flow of ATF flowing through a friction interface and by forming grooves at the friction interface.
As an example, Japanese Patent Laid-Open Publication No. 2001-295859 (“JP2001-295859”) discloses a wet friction material comprising a core plate and a plurality of segment pieces stuck at intervals on the core plate in a ring shape. In JP2001-295859, lubricating oil grooves are defined between two adjacent segment pieces or formed respectively by facing lateral end surfaces of the adjacent segment pieces. Thus, the lubricating oil grooves connect an inner peripheral side and an outer peripheral side of the core plate. In JP2001-295859, at least one of the facing lateral end surfaces of the segment piece has a cut-in part at an inner peripheral opening portion and/or a middle part thereof. Thus, the lubricating oil groove includes a large width part of a large groove width that is formed by the cut-in part at the inner peripheral opening portion and/or the middle part thereof.
More specifically, in addition that the lubricating oil groove is sectioned and formed by the facing lateral end surfaces (or opposite end surfaces) of the segment piece, the cut-in part is provided on the inner peripheral end portion and/or the middle part of the opposite end surfaces of the segment piece. Consequently, the opposite end surface is formed into a non-linear shape, and the groove width of the lubricating oil groove is not constant. Thus, the lubricating oil groove has a non-large width part of a groove width smaller than that of the large width part, in addition to the large width part of the large groove width. As described above, if the lubricating oil grooves have the large width part and the non-large width part by forming the cut-in part dented inward on the segment piece, ATF running from the inner peripheral side toward the outer peripheral side of the lubricating oil groove is blocked at a point where the width of the lubricating oil groove is changed from the large width part to the non-large width part. Then, a part of ATF overflows and runs over top surfaces of the segment pieces.
Japanese Patent Laid-Open Publication No. 2005-282648 (“JP2005-282648”) discloses a wet friction material comprising a plurality of lubricating oil grooves. The lubricating oil groove is formed by a clearance between adjacent ones of segment pieces. Alternatively, the lubricating oil groove is formed by applying press working on a friction member that is provided on each of opposite surfaces or on one surface of a metal core plate. The plurality of lubricating oil grooves are composed of two kinds of lubricating oil grooves (first lubricating oil grooves and second lubricating oil grooves). In the first lubricating oil groove, an inner peripheral opening portion or a middle part thereof is bulged or enlarged so that the lubricating oil groove is shaped into a symmetrical shape. The second oil groove has an almost uniform width from an inner peripheral opening portion to an outer peripheral opening portion. The first and the second lubricating oil grooves are mixed at a predetermined ratio. When the wet friction material runs idle in a disengaged state in either direction, ATF supplied from an inner periphery comes into contact with a first portion (of the bulged two sides) of the lubricating oil groove that is enlarged toward an idling rear side. Consequently, ATF is actively supplied to a friction surface of a friction material substrate to restrain contact between a counter plate and the friction surface of the friction material substrate. Thus, excess ATF is discharged by the oil groove having the almost uniform width from the inner peripheral opening portion to the outer peripheral opening portion thereof. As a result, it is possible to obtain a remarkable effect to reduce the drag torque. On the other hand, when the wet friction material runs idle in a reverse direction, a second portion (of the bulged two sides) at an opposite side of the oil groove plays the same role as the first portion. If the wet friction material is embodied into a segment-type friction material, the segment pieces can be formed into a large size that extends laterally and that occupies nearly a full width of a ring shape of the metal core plate. Therefore, the number of the segment pieces can be reduced, and the time for cutting out and sticking the segment pieces can be shortened, so that cost reduction can be achieved. Moreover, if the wet friction material is embodied in a press-type friction material, it can be produced only by sticking a friction material substrate on both surfaces or one surface along an entire circumference of the ring shape of the metal core plate and by pressing both surfaces or one surface thereof. Therefore, cost reduction can be achieved by mass production.
In addition, Japanese Patent Laid-Open Publication No. 2010-101402 (“JP2010-101402”) discloses a wet friction material as a segment-type friction material or a ring-type friction material. In the segment-type friction material, a plurality of segment pieces is prepared by cutting a friction material substrate into a segment shape. Then, the segment pieces are attached via an adhesive to one side or opposite sides of a flat ring-shaped metal core along an entire circumference of the metal core. At this time, the segment pieces have their outer circumferential edges located inward by a predetermined amount from an outer periphery of the metal core to provide a clearance area of such predetermined amount or width therebetween. One or plural protrusions is/are provided on the outer peripheral side of the segment pieces (i.e. at the clearance portion be) so as to protrude toward the outside. In the ring-type friction material, a ring-shaped piece is prepared by cutting a friction material substrate into a ring shape. Then, the ring-shaped piece is attached via an adhesive to one side or opposite sides of the metal core along an entire circumference of the metal core. At this time, the ring-shaped piece has an outer circumferential edge thereof located inward by a predetermined amount from an outer periphery of the metal core to provide a clearance area of such predetermined amount or width therebetween. One or plural protrusions is/are provided on the outer peripheral side of island shaped portions of the ring-shaped piece so as to protrude toward the outside.
As described above, the protrusion(s) is/are provided to project toward and inside the clearance area. Consequently, if a flow of ATF is accumulated in an outer peripheral portion by a centrifugal force in a region of relatively high rotating speed, such flow is controlled by the protrusion(s). Thus, a flow of ATF is prevented from being disturbed by a high-rotation speed. As a result, ATF is kept from running over the segment pieces or the island shaped portions. Thereby, there is attained a great effect to reduce a drag torque even in the region of relatively high rotating speed. Thus, there is provided a wet friction material that is a segment-type friction material or a ring-type friction material and that obtains an excellent effect to reduce the drag torque even in the region of relatively high rotating speed by controlling excess flow of ATF flowing from the clearance area to the segment pieces or the island shaped portions.
According to JP2001-295859, ATF running from the inner peripheral side toward the outer peripheral side of the oil grooves is blocked at the point where the width of the oil grooves is changed from the large width part to the non-large width part. Then, a part of ATF overflows and runs over the top surface of the wet friction material. Thereby, the drag toque can be reduced.
The wet friction material according to JP2005-282648 has two types of lubricating oil grooves mixed at the predetermined ratio: one having the almost uniform width from the inner peripheral opening portion thereof to the outer peripheral opening portion thereof and the other having non-uniform width. Thus, when the wet friction material runs idle in the disengaged state in either direction, ATF supplied from the inner periphery is actively supplied to the friction surface of the friction material substrate. As a result, the contact between the counter plate and the friction surface is restrained.
However, in the inventions described in JP2001-295859 and JP2005-282648, ATF running along an inner peripheral end face and an outer peripheral end face of the segment piece comes into contact with the inner peripheral end face and the outer peripheral end face of the segment piece. Therefore, it is hard to reduce a drag torque generated by a resulting contact resistance or shearing torque.
In the wet friction material according to the JP2010-101402, the protrusions control the flow of ATF that is accumulated in the outer peripheral portion by the centrifugal force in the region of relatively high rotating speed. Thus, it is prevented that the flow of ATF is disturbed by the high-speed rotation thereby to make ATF run over the segment pieces or the island shaped portions. Thereby, an effect to reduce the drag torque can be obtained even in the region of relatively high rotating speed.
However, the invention of the JP2010-101402 aims to control excess ATF flowing over the segment pieces or the island shaped portions. With respect to a drag torque generated by contact between ATF and an inner peripheral end face and an outer peripheral end face of the segment piece, it is hard to obtain a sufficient effect to reduce the drag torque as in the inventions described in JP2001-295859 and JP2005-282648.
In the conventional techniques, though not limited to JP2001-295859, JP2005-282648 and JP2010-101402, a plurality of segment friction materials is stuck on a core plate in a ring shape at intervals. A counter plate faces the segment friction materials. The segment friction materials aim to reduce a drag toque, while keeping a contact area with the counter plate as a counter member as large as possible, in order to assure a desired torque transmission.
In view of the above, it is an object of the present invention to provide a wet friction material that controls a contact resistance or a shearing torque between inner and outer peripheral end faces of the wet friction material and ATF and that reduces a drag torque efficiently.